Method for producing an anti-corrosion coating for hardenable sheet steels and an anti-corrosion coating for hardenable sheet steels

ABSTRACT

The invention relates to a method for producing an anti-corrosion coating for hardenable sheet steels, wherein at least two metal layers are deposited one after another onto the steel substrate; the one metal layer is a zinc layer or zinc-based layer and the other layer is a layer composed of a metal that forms baser intermetallic phases with Zn or Fe and has a higher oxidation potential than Zn, namely Ni, Cu, Co, Mn, or Mo, or a layer based on these metals; and an anti-corrosion coating for hardenable sheet steels.

FIELD OF THE INVENTION

The invention relates to a method for producing an anti-corrosion coating for hardenable sheet steels and to an anti-corrosion coating for hardenable sheet steels.

BACKGROUND OF THE INVENTION

There are currently two common methods for hardening sheet steels and for producing hardened sheet steel components out of sheet steel, in particular vehicle body components.

The two methods share the fact that a steel band is produced from a steel material by means of hot rolling, usually also followed by cold rolling and the steel band is then continuously galvanized. The usual galvanizing method in this case is hot-dip galvanizing in which the steel band is conveyed through a vat of molten zinc; the molten zinc adheres to the steel, the galvanized steel band is usually conveyed vertically from the vat after which the excess zinc is stripped by means of stripping jets, and then the band undergoes a heat treatment as needed. The resulting galvanized steel band is then usually shipped in coils, i.e. in wound form.

In order to then produce hardened sheet steel components out of this steel band, blanks of a desired size are stamped from the steel band and these blanks are then processed further in two different ways.

In a first method, the blanks are shaped in an intrinsically conventional manner in a multi-stage process and in particular, are deep drawn, until the component has been shaped into its final form. In this case, however, the component is usually about 2% smaller in all three spatial directions in order to take into account a subsequent thermal expansion. Then this sheet metal component is heated to an austenitization temperature, i.e. a temperature above Ac₃, and possibly kept there until the steel material is in the austenitic phase. Then the heated sheet steel component is transferred to a form-hardening die and in the form-hardening die, into which the heated sheet steel component can usually be inserted in a form-fitting manner, is held in a pressed way—without being appreciably shaped—by a female die and a male die. Through the contact with the female die and male die, which can also be cooled, the steel component is cooled at a speed that is greater than the critical hardening speed, which results in a conversion of the austenite essentially to martensite and yields a high hardness of the component.

In a second known method, the blank is directly heated to a temperature above Ac₃ that is necessary for hardening and if need be, is kept there and then shaped in a one-step stroke in a tool composed of a female die and male die, and, by means of the contact of the tool with the work piece, is simultaneously cooled quickly enough so that the hardening outlined above occurs. This method is referred to as press-hardening.

Form-hardening is superior to press-hardening when it comes to the possible geometries of a component because more complicated or complex three-dimensional forms can be achieved in a multi-step shaping process, whereas during the one-step shaping press-hardening, only comparatively simple geometries can be achieved.

The final result of both methods, however, is a hardened sheet steel component.

Usual materials for these sheet steel components are so-called boron-manganese steels, in particular the most commonly used 22MnB5.

It is known that particularly with the press-hardening method, problems can occur such that at high temperatures, molten zinc experiences interactions with the austenite in the steel material that are not yet fully understood, but that result in the formation of cracks in the regions that have undergone intense shaping. This phenomenon is referred to as so-called “liquid metal embrittlement.”

Attempts have already been made to counteract this phenomenon through the use of transformation-delayed steel types, which are austenitized at a higher temperature, then intercooled and by means of this intercooling, reach temperatures that lie below the melting temperature of zinc phases in the coating, and only then is the shaping carried out. Due to the transformation delay, the iron still exists in the form of austenite even at these relatively low temperatures so that a reliable quench hardening can be achieved.

DE 10 2010 030 465 A1 has disclosed a method for producing a shaped sheet metal component provided with an anti-corrosion coating and composed of a higher-strength sheet steel material. This method includes the steps of shaping a prepared initial sheet metal material into a shaped sheet metal component and producing the anti-corrosion coating by electrolytically depositing a zinc-nickel coating onto the shaped sheet metal component; at the beginning of the coating process, first a thin nickel layer is deposited, which in the succeeding steps, prevents a hydrogen embrittlement of the sheet steel material. It also discloses a hot-formed and in particular press-hardened shaped sheet metal component composed of a higher-strength sheet steel material with an electrolytically deposited zinc-nickel coating. The point of this is to provide the nickel layer as a barrier against hydrogen that is typically introduced into the sheet steel material during the electrolytic coating process.

A method for producing a steel component provided with a metallic coating that protects against corrosion and the steel component itself are known from EP 2 290 133 B1. The aim is to create a method that is simple to carry out in practice, which makes it possible—with a comparatively small amount of effort—to produce a steel component provided with a favorably adhering metallic coating that reliably protects against corrosion since, as is explained, zinc coatings do not adhere well to the sheet steel types used for hot press-hardening. Furthermore, known coatings have a poor paint adhesion due to oxidation of the surface. According to this document, the applied anti-corrosion layer should be an electrolytically deposited γ-ZnNi phase, which should favorably withstand subsequently performed heating operations for purposes of austenitization.

EP 0 364 596 B1 relates to a method for producing zinc-nickel alloy-coated thin sheets with good press-deformation properties, the purpose being to improve the shaping capacity of such sheets by means of a zinc-nickel alloy coating. The layer in this case is to be deposited with approx. 30 g/m² and a nickel content of 12.5%.

The object of the invention is to create a method for producing hardened sheet steel components.

Another object of the invention is to create an anti-corrosion coating for hardenable sheet steels, which, while having good cathodic corrosion protection, reduces or even prevents liquid metal embrittlement.

SUMMARY OF THE INVENTION

According to the invention, an at least two-layered anti-corrosion layer is produced on a steel sheet; in this case, either a very thin 1 μm nickel layer is electrolytically deposited onto the steel and then a zinc layer is likewise electrolytically deposited onto the nickel layer, or the thin nickel layer is formed by means of an electrolytic deposition onto the steel sheet and then a zinc layer is applied by means of hot-dip galvanization. Another possibility is to apply a nickel-containing layer onto a normal hot-dip galvanized sheet steel band by means of a corresponding aftertreatment (coater).

According to the invention, the nickel layer is approx. 1 μm thick if it is applied as a first layer by means of electrolytic deposition.

If a hot-dip galvanized zinc layer undergoes aftertreatment, then the outer nickel-containing layer is approx. 250 nm to 700 nm thick.

According to the invention, it has surprisingly turned out that with the coating according to the invention and the layer structure according to the invention, the nickel does not in any way constitute a barrier against the ability of the molten zinc to come into contact with the steel; instead, the nickel appears to react very quickly with the zinc and also iron so that the melting point of the entire anti-corrosion layer increases abruptly since instead of zinc-iron Γ phases, an increased amount of zinc-nickel-iron phases are formed, which have a significantly higher melting point. This results in the fact that at the temperatures at which the hot-forming and quench-hardening take place, no molten phases are present that could interact with the austenite. This is also the reason why according to the invention, an outer applied nickel layer yields a comparable effect; the nickel, which is deposited onto the outermost surface, diffuses into the anti-corrosion layer so quickly that it assures the increase in the melting point.

According to the invention, instead of nickel or nickel-based layers, other elements, which form baser intermetallic phases with Zn or Fe and have a higher oxidation potential than Zn, for example Cu, Co, Mn, or Mo, can be used since the same effects are achieved by manganese, molybdenum, cobalt, and copper. In this connection, the expression “x-based,” where x is an element, means that these elements make up the majority (>50 wt %), but other elements are present as alloying elements.

Nickel and cobalt as well as manganese or copper do not act as physical barriers against the diffusion between zinc and iron, but are dissolved in and incorporated into the molten zinc and zinc-iron phases. With a previously applied nickel layer and a subsequent hot-dip galvanization, the molten zinc at least begins to dissolve the nickel during the galvanization process.

With a standard annealing for purposes of austenitization and subsequent shaping, it has been possible to determine that a phase structure of the layer forms, which is similar to that of pure hot-dip galvanized layers (phs-ultraform); this phase structure, however, is richer in zinc and has a higher percentage of Γ phases. The fact that these phases are richer in zinc is advantageous for the cathodic corrosion protection capacity of the layer.

This is also promoted by the fact that a very zinc-rich layer forms close to the surface, a phenomenon that is not observed in conventional hot-dip galvanization layers in which standard annealing without an intermediate nickel layer is used.

The positive effect of the nickel in the layer or as a separately applied electrolytic layer becomes apparent when bending samples are observed. The formation of cracks due to liquid metal embrittlement is reduced to an impressive degree.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be explained by way of example based on the drawings. In the drawings:

FIG. 1: shows a light microscopically etched micrograph of a steel sheet with the coating according to the invention in which a hot-dip galvanized layer has been applied onto a 1 μm-thick intermediate nickel layer;

FIG. 2: shows an enlarged depiction of the layer according to FIG. 1;

FIG. 3: shows a layer according to FIG. 1 in which EDX element mapping has been used to depict the distribution of the elements iron, zinc, nickel, and aluminum in an intermediate nickel layer that has been applied 1 μm thick;

FIG. 4: shows a micrograph of the coating with a 0.5 μm-thick nickel layer and a 10 μm-thick zinc layer that has been annealed at 800° C.;

FIG. 5: shows the layer according to FIG. 4 with a 1 μm-thick nickel layer;

FIG. 6: shows a coating according to the invention after the annealing, a holding time, a transfer time, and a subsequent cooling for press-hardening purposes;

FIG. 7: shows an X-ray microscopic micrograph of an anti-corrosion layer according to the invention, after an austenitic annealing at 870° C.;

FIG. 8: shows the layer according to FIG. 7 with the distribution of the iron;

FIG. 9: shows the layer according to FIG. 7 with the distribution of the zinc;

FIG. 10: shows the layer according to FIG. 7 with the distribution of the nickel in which a nickel support layer has been applied to the surface as a preparation aid;

FIG. 11: shows the layer according to FIG. 7 with the distribution of the aluminum;

FIG. 12: shows the layer according to FIG. 7 with the distribution of the manganese;

FIG. 13: shows a coating according to the invention after the austenitization and quenching, with an EDX scan line indicated;

FIG. 14: shows the coating according to FIG. 13, with the scan profile for the elements iron, nickel, and zinc;

FIG. 15: shows four V samples with a bending radius of 1.5 mm.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The method according to the invention for producing sheet steel components can either be a press-hardening method or a form-hardening method, i.e. a method in which a sheet steel component is heated and then quench-hardened in a tool (form-hardening) or a method in which a blank is shaped and quench-hardened in a single step (press-hardening).

According to the invention, a boron-manganese steel is used as the steel material for the press-hardening or form-hardening and with regard to the transformation of the austenite into other phases, the transformation can be shifted to lower ranges and martensite is formed.

In particular the alloying elements boron, manganese, carbon, and optionally chromium and molybdenum are present in steels of this kind as transformation delayers, i.e. as an element that shifts the phase transformation of austenite into martensite to lower temperatures.

Steels with the following general alloy composition are suitable for the invention (all amounts indicated in percentage by weight):

carbon (C) 0.08-0.6  manganese (Mn) 0.8-3.0 aluminum (Al) 0.01-0.07 silicon (Si) 0.01-0.5  chromium (Cr) 0.02-0.6  titanium (Ti) 0.01-0.08 nitrogen (N) <0.02 boron (B) 0.002-0.02  phosphorus (P) <0.01 sulfur (S) <0.01 molybdenum (Mo) <1 traces of iron and smelting-related impurities.

The following steel configurations have turned out to be particularly suitable (all amounts indicated in percentage by weight):

carbon (C) 0.08-0.34 manganese (Mn) 1.00-3.00 aluminum (Al) 0.03-0.06 silicon (Si) 0.01-0.20 chromium (Cr) 0.02-0.3  titanium (Ti) 0.03-0.04 nitrogen (N) <0.007 boron (B) 0.002-0.006 phosphorus (P) <0.01 sulfur (S) <0.01 molybdenum (Mo) <1 traces of iron and smelting-related impurities.

The conventional steels 22MnB5 and 20MnB8 are thus particularly also suitable.

By adjusting the alloying elements that function as transformation delayers, a quench-hardening, i.e. the rapid cooling at a cooling speed that is greater than the critical hardening speed, is still reliably achieved below 780° C. This means that in this case, processing is carried out below the peritectic temperature of the zinc-iron system, i.e. mechanical stresses are only exerted below the peritectic temperature. This also means that at the moment in which mechanical stresses are exerted, there are no longer any molten zinc phases that can come into contact with austenite.

An anti-corrosion layer according to the invention is an anti-corrosion layer that is applied in at least two layers, with at least one nickel layer and one zinc layer being applied to a substrate composed of a hardenable steel material. Instead of a nickel layer, it is also possible to apply a manganese or copper layer.

In this case, the nickel, copper, or manganese layer is preferably deposited electrolytically. The zinc layer can be electrolytically deposited or deposited by means of a hot-dip method.

Basically, it is possible to apply the nickel layer first and then to apply a zinc layer, with the subsequently applied zinc layer being electrolytically deposited or deposited by means of a hot-dip method.

Another possibility lies in depositing the zinc layer electrolytically or by means of a hot-dip method as a first layer and then applying a nickel layer on the outermost layer thereof, in particular by means of electrolytic deposition.

Wherever the term “nickel” is used here, this is also meant to include other elements that form baser intermetallic phases with Zn or Fe and that have a higher oxidation potential than Zn, for example Cu, Co, Mn, or Mo.

The element nickel in this case is also used as a proxy for copper and manganese.

It has surprisingly turned out that correspondingly applied metal layers clearly intervene in the phase structure of an anti-corrosion layer, but do not themselves constitute diffusion barriers. Consequently, the invention is even effective if the thin nickel, copper, or manganese layer is applied onto a hot-dip galvanization layer.

FIG. 1 shows a light microscopically etched micrograph of the layer according to the invention on a steel substrate. FIG. 2 shows it again, enlarged even further.

In this layer, first a 1 μm-thick intermediate nickel layer is applied to the steel substrate and then hot-dip galvanized, with the intermediate nickel layer having been dissolved in the zinc bath in the hot-dip galvanization.

When a layer of this kind is measured with an EDX element mapping, FIG. 3 clearly shows that a homogeneous distribution of iron is present in the region of the steel, with the iron content decreasing at the boundary layer with the layer applied on top of it. It is clear from the distribution of zinc that the zinc content increases in the boundary zone with the higher layers.

With regard to nickel, it is clear that there must be a homogeneous distribution within the layer because there is no clear, colored evidence of the nickel. The same is true for aluminum, which is contained in the zinc coating for the hot-dip galvanization.

FIGS. 4 and 5 both compare respective layers, in one of which, the intermediate nickel layer had a thickness of 0.5 μm (FIG. 4) and in the other of which, it had a layer thickness of 1 μm (FIG. 5). A hot-dip galvanized layer 10 μm thick was then deposited onto this. Both layer samples were then heated to 800° C. The uppermost, light-colored layer is not part of the anti-corrosion layers; it is a preparation aid layer composed of nickel, which was applied before the sample preparation, i.e. after the heating and cooling.

In coatings according to the invention, a structure that has two phases in the micrograph forms on the surface of the steel substrate, with a light-colored phase, which is interspersed with dark areas (FIG. 6). In this case, with a 1 μm-thick intermediate nickel layer, annealing was performed at 870° C., followed by a waiting period of 45 s, then a transfer time of 5 s, and then a cooling in a press.

A layer according to FIG. 6 was measured with an EDX element mapping; here, too, a preparation aid in the form of a nickel support layer is present on the sample. The layer section that was measured is shown in FIG. 7.

In the distribution of the iron (FIG. 8), it is clear that relatively little iron is present in the light-colored phase, while the dark phase exhibits a significant iron content.

It is clear that the zinc is highly concentrated in the light-colored phase whereas it is present in much lower concentrations in the dark areas so that an iron-rich phase in the form of a nodule or roundish concentration in a zinc matrix is clearly present.

The nickel (FIG. 10) is still not very visible in the light-colored zinc matrix, but is clearly not present in the iron-rich nodules, whereas aluminum (FIG. 11) is distributed in a relatively homogeneous fashion throughout the entire layer, even though there are concentrations in the iron-rich phases.

Manganese, which is present in the base steel material, is hardly present at all in the entire layer and is detectable only in the substrate.

In a comparable layer, the distribution of elements in the depth direction was determined using a so-called EDX line scan (FIG. 13). In this case, the scan begins already in the nickel support layer and extends deep into the steel base material.

In a coating that was annealed at 800° C., had a transfer time of 5 s, and was then press-cooled, this yields a corresponding distribution of elements, as is clear in FIG. 14. At the beginning, the nickel peak is almost 100%, which is due to the fact that the scan already begins in the nickel preparation layer. Then the nickel content decreases; it is clear that in the dark-colored iron-rich zones, the nickel content is significantly lower than in the light-colored, zinc-rich phases. Thus starting from the outer surface, the iron content is very low at approx. 10% and increases significantly in the dark-colored iron-rich phase, then reaching its maximum in the steel matrix. The zinc content behaves inversely to the iron content, which was to be expected given the two-phase structure.

An originally present nickel layer is no longer detectable in the phase structure.

The positive effect of the nickel in the layer is clear from bending samples with a radius of 1.5 mm (FIG. 15).

Whereas with an intermediate nickel layer of only 0.5 μm (due to the complete dissolution of the nickel) in the anti-corrosion coating, the thickness of the nickel layer only influences the quantity of the nickel in the layer. With 0.5 μm nickel and 10 μm hot-dip galvanized layer on top of it, an annealing temperature of 870° C., a holding time of 45 s, a transfer time of 6 s, and corresponding cooling in the press, a crack pattern of the kind shown in the two depictions on the left in FIG. 15 is produced.

By contrast, with a 1 μm-thick nickel layer and the same conditions otherwise, a very much finer crack pattern is produced (the two depictions on the right in FIG. 15).

The invention thus makes it possible, by means of an additional nickel layer, to exert influence on the zinc-based anti-corrosion layer such that during the cooling, this layer clearly forms solid phases more quickly, which then do not react with the austenite of the steel substrate during shaping.

Particularly in the zinc-rich light-colored phases of the coating, more nickel dissolves than in the dark-colored iron-rich phases, which intrinsically have a higher melting point.

By comparison with a zinc-nickel layer deposited exclusively by means of electrolysis, the invention has the advantage that it enables a mixed deposition using both an electrolytic method and a hot-dip coating method. In addition, the nickel layer can be easily applied to conventional sheets that have already been hot-dip galvanized; this can be done with both an electrolytic coating method and other coating methods, e.g. roller application, i.e. a method that uses rollers for application, for example a coil-coating method in which a nickel-containing layer with a thickness of 250 nm to 700 nm is applied. 

1. A method for producing hardened sheet steel components, comprising: depositing at least two metal layers, one after another, onto a steel substrate, wherein the at least two metal layers act as an anti-corrosion coating and form a band composed of a quench-hardenable steel alloy; a first metal layer is a zinc layer or zinc-based layer and a second metal layer is a layer composed of a metal that forms baser intermetallic phases with Zn or Fe and has a higher oxidation potential than Zn, namely Ni, Cu, Co, Mn, or Mo, or a layer based on these metals; and the steel substrate has the following general alloy composition, respectively indicated in percentage by weight: carbon (C) 0.08-0.6 manganese (Mn)  0.8-3.0 aluminum (Al)  0.01-0.07 silicon (Si) 0.01-0.5 chromium (Cr) 0.02-0.6 titanium (Ti)  0.01-0.08 nitrogen (N) <0.02 boron (B) 0.002-0.02 phosphorus (P) <0.01 sulfur (S) <0.01 molybdenum (Mo) <1

traces of iron and smelting-related impurities; stamping blanks from the band provided with the anti-corrosion coating; and either heating the blanks to a temperature>Ac₃ and keeping the blanks at this temperature if need be, and then shaping the blanks in a press-hardening tool and quench-hardening the blanks in order to produce the sheet steel component; or cold forming the blanks into a sheet steel component and then heating the sheet steel component to a temperature>Ac₃ and quench-hardening the sheet steel component in a form-hardening die.
 2. The method according to claim 1, comprising using a material with the following alloy composition as the steel substrate, respectively indicated in percentage by weight: carbon (C) 0.08-0.34 manganese (Mn) 1.00-3.00 aluminum (Al) 0.03-0.06 silicon (Si) 0.01-0.20 chromium (Cr) 0.02-0.3  titanium (Ti) 0.03-0.04 nitrogen (N) <0.007 boron (B) 0.002-0.006 phosphorus (P) <0.01 sulfur (S) <0.01 molybdenum (Mo) <1

traces of iron and smelting-related impurities.
 3. The method according to claim 1, comprising applying the zinc layer or zinc-based layer electrolytically or using a hot-dip method.
 4. The method according to claim 1, comprising applying the nickel, copper, or manganese layer electrolytically or using a roller application method.
 5. The method according to claim 1, comprising applying the nickel, copper, or manganese layer with a thickness of 0.5 μm to 2 μm with electrolytic deposition or with a thickness of 250 nm to 700 nm with roller application.
 6. The method according to claim 3, comprising depositing the zinc layer or zinc-based layer with a thickness of 6 μm to 30 μm.
 7. The method according to claim 1, comprising first depositing the layer composed of nickel, copper, or manganese onto the steel substrate and then depositing the zinc layer or zinc-based coating onto the steel substrate.
 8. The method according to claim 1, comprising depositing the zinc coating or zinc-based coating onto the layer composed of nickel, copper, or manganese electrolytically or using hot-dip galvanization.
 9. The method according to claim 1, comprising first applying the zinc layer or zinc-based layer to the steel substrate electrolytically or using a hot-dip coating method and then applying the nickel layer to the zinc layer electrolytically or applying the nickel layer to the zinc layer using a roller application method.
 10. The method according to claim 1, comprising repeatedly applying the layer sequence, alternating between the nickel, copper, and manganese layer and the zinc or zinc-based layer.
 11. An anti-corrosion layer for use in the method according to claim 1, the anti-corrosion layer comprising at least two layers, one layer is present that is composed of nickel, copper, or manganese and on top of or underneath it, a zinc layer or zinc-based layer is present.
 12. The anti-corrosion layer according to claim 11, wherein the zinc layer or zinc-based layer is deposited electrolytically or using a hot-dip method.
 13. The anti-corrosion layer according to claim 11, wherein the nickel, copper, or manganese layer is applied electrolytically or using a roller application method.
 14. The anti-corrosion layer according to claim 11, wherein the nickel, copper, or manganese layer has a thickness of 0.5 μm to 2 μm with electrolytic deposition or a thickness of 250 nm to 700 nm with a roller application method.
 15. The anti-corrosion layer according to claim 11, wherein the zinc layer or zinc-based layer has a thickness of 6 μm to 30 μm.
 16. The anti-corrosion layer according to claim 11, wherein the layer composed of nickel, copper, or manganese is positioned on the steel substrate and the zinc layer or zinc-based coating is positioned on top of it.
 17. The anti-corrosion layer according to claim 11, wherein a zinc layer or zinc-based coating, which has been deposited electrolytically or using hot-dip coating, is applied to the steel substrate and the nickel layer is positioned on the zinc layer, the nickel layer being applied electrolytically or using a roller application method.
 18. The anti-corrosion layer according to claim 11, wherein a repeated sequence of the layers nickel, copper, and manganese on the one hand and zinc or zinc-based layers on the other is present on the steel substrate. 